Rod-cone interaction will be studied in human subjects by means of psychophysical procedures, and in the tiger salamander retina by means of intracellular and extracellular neurophysiological recording. Both types of research will be used to test a quantitative model which accounts for rod-cone interaction by a summatory process. The related research will critically examine the importance of spatial, temporal, and illuminace-related variables upon the extent of rod-cone interaction. Additionally, several psychophysical experiments will be performed with color-defective subjects in order to assess the types of cone mechanisms which interact with rods. One neurophysiological study will separately examine the influence of both rods and cones upon the ionic channels of horizontal cells and biopolar cells.